Practical problems in the production of fine lines on circuit boards

With the development of the electronics industry, the integration of electronic components is getting higher and higher, while the size is getting smaller and smaller, and BGA type packaging is widely used. Therefore, the lines of PCBs will become smaller and smaller, and the number of layers will increase. Reducing the line width and line spacing is to make the best use of the limited area, and increasing the number of layers is to make use of space. The mainstream of the lines of future circuit boards will be 2-3mil, or smaller.

It is generally believed that every time the production of circuit boards increases or rises to a higher level， an investment must be made, and the investment funds are large.

In other words, high-end circuit boards are produced by high-end equipment. However, not every company can afford large-scale investment, and after the investment, it takes a lot of time and money to conduct experiments to collect process data and trial production. It seems to be a better method to conduct experiments and trial production according to the current situation of the company, and then decide whether to invest according to the actual situation and market conditions. This article details the limit of the width of fine lines that can be produced under normal equipment conditions, as well as the conditions and methods for the production of fine lines.

. The circuit obtained by acid etching is very uniform, which is conducive to impedance control and has less environmental pollution, but a hole will cause scrapping; alkaline etching production control is relatively easy, but the circuit is uneven and the environmental pollution is also large.

The general production process can be divided into the cap hole acid etching method and the graphic electroplating method, each of which has its advantages and disadvantages

First of all, the first thing to make the circuit is the dry film. Different dry films have different resolutions, but generally they can show a line width and line spacing of 2mil/2mil after exposure. The resolution of ordinary exposure machines can reach 2mil, and generally there will be no problems with the line width and line spacing within this range. The nozzle, pressure, and concentration of the developing machine are not very related to the line width and line spacing of 4mil/4mil or above. Below 3mil/3mil line width and line spacing, the nozzle is the key to affecting the resolution. Generally, a fan-shaped nozzle is used, and the pressure can be developed at about 3BAR.

Although the exposure energy has a great impact on the circuit, most of the dry films currently used on the market generally have a wide exposure range

It can be distinguished at 12-18 levels (25-level exposure scale) or 7-9 levels (21-level exposure scale).

Generally speaking, lower exposure energy is beneficial to resolution, but when the energy is too low, dust and various debris in the air have a great impact on it, which will cause open circuit (acid etching) or short circuit (alkali etching) in the subsequent process. Therefore, in actual production, it should be combined with the cleanliness of the darkroom, so that the minimum line width and line spacing of the circuit board that can be produced can be selected according to the actual situation.

The impact of development conditions on resolution is more obvious when the line is smaller.

When the line is above 4.0mil/4.0mil, the development conditions (speed, solution concentration, pressure, etc.) have no obvious impact; the line is 2.0mil/2.0/mil When the circuit is developed, the shape and pressure of the nozzle play a key role in whether the circuit can be developed normally. The development speed may drop significantly at this time. At the same time, the concentration of the solution affects the appearance of the circuit. The possible reason is that the fan-shaped nozzle has a large pressure. When the circuit spacing is very small, the impact can still reach the bottom of the dry film, so it can be developed; the conical nozzle has a smaller pressure, so it is difficult to develop fine circuits. In addition, the direction of the board has a significant impact on the resolution and the side wall of the dry film.
Different exposure machines have different resolutions. The exposure machines currently used are one type of air-cooled, surface light source, and the other is water-cooled, point light source. The nominal resolution is 4mil. However, experiments have shown that without special adjustments or operations, 3.0mil/3.0mil can be achieved; even 0.2mil/ 0.2/mil; 1.5mil/1.5mil can also be distinguished when the energy is reduced, but the operation must be careful at this time, and the dust and debris have a great impact. In addition, there is no obvious difference in the resolution of the Mylar surface and the glass surface in the experiment.

For alkaline etching, there is always a mushroom effect after electroplating, which is generally only distinguished between obvious and unobvious. If the circuit is larger than 4.0mil/4.0mil, the mushroom effect is smaller.

When the circuit is 2.0mil/2.0mil, the impact is very large. The dry film forms a mushroom shape due to the overflow of lead and tin during electroplating. The dry film is sandwiched inside, making it very difficult to remove the film. The solutions are: 1. Use pulse electroplating to make the coating uniform; 2. Use a thicker dry film, generally 35-38 The thicker dry film is 50-55 microns, which is more expensive. This dry film works better in acid etching; 3. Use low current electroplating. But these methods are not thorough. In fact, it is difficult to have a very complete method.

Because of the mushroom effect, it is very troublesome to remove the film of fine lines.

Since the corrosion of lead and tin by sodium hydroxide is very obvious at 2.0mil/2.0mil, it can be solved by thickening the lead and tin during electroplating and reducing the concentration of sodium hydroxide.

Different line widths have different speeds during alkaline etching, and different line shapes have different speeds. If the circuit board has no special requirements for the thickness of the line, the circuit board with a copper foil thickness of 0.25oz is used or a part of the base copper of 0.5oz is etched away, the electroplated copper is thinner, and the lead and tin are thickened. It is effective for making fine lines with alkaline etching. In addition, the nozzle needs to be fan-shaped. Conical nozzles can generally only reach 4.0mil/4.0mil.

The same thing as alkaline etching is that the line width and line shape speed are different in acid etching, but generally, when using acid etching, the dry film is easy to break or scratch the masking film and the surface film during the transmission and the previous process, so be careful during production. The line effect of acid etching is better than that of alkaline etching, and there is no mushroom effect. The side etching is less than that of alkaline etching. In addition, the fan-shaped nozzle is significantly better than the conical nozzle. The impedance change of the line after acid etching is smaller.

In the production process, the speed and temperature of the film, the cleanliness of the board surface, and the cleanliness of the diazo film have a greater impact on the qualified rate. The parameters of the acid etching film and the flatness of the board surface are particularly important; for alkaline etching, the cleanliness of the exposure is very important.

Therefore, it is believed that ordinary equipment can produce 3.0mil/3.0mil (referring to the film line width and spacing) boards without special adjustments; but the qualified rate is affected by the environment and the proficiency and operation level of the personnel. Alkaline etching is suitable for the production of circuit boards below 3.0mil/3.0mil, unless the base copper is small to a certain extent, the fan-shaped nozzle is significantly better than the conical nozzle.